Ordinary storage boilers, such as those for domestic use, may be ‘single’, i.e. comprise one tank and one heater, or ‘double’, i.e. comprise two storage boilers in series and one inside the other, such as those described in DE3218442 or US2004/0079749. In the latter case, the storage boiler comprises a main tank with a first heater for heating the water to, and keeping it at, a given standby temperature T1; and a smaller secondary tank inserted inside and insulated thermally from the main tank, and having a second heater for heating the water to, and keeping it at, a given second standby temperature T2 higher than T1. Another example of a ‘double’ storage boiler is described in CH367610, in which the second heater is only activated to produce steam; in which case, in response to the increase in pressure inside the secondary tank, a valve cuts off the fluid connection between the two tanks, so that only the water in the secondary tank is converted to steam.
In known storage boilers used in hot-beverage vending machines, the heating means usually comprise one or more electric resistors for heating the water in the tank to, and keeping it at, a standby temperature equal to the temperature at which the beverage is dispensed, and which is normally close to 100° C., but varies from one beverage to another.
Known storage boilers of the above type have numerous drawbacks, foremost of which are:                a relatively high standby temperature, which, both in itself and because of the pressures generated, rules out plastic as a manufacturing material for the tank, which is normally made of metal, preferably steel;        poor thermal efficiency, due to severe heat loss to the outside caused by the relatively high standby temperature and the metal from which the tank is made;        poor versatility, due to the impossibility, even by modulating the electric resistors, of altering the temperature of all the water in the tank relatively quickly; as a result, a vending machine for producing different hot beverages at different temperatures must normally be equipped with a number of boilers, preferably one for each type of beverage.        